Unclear behaviour of param +nadgrid= vs. +nadgrids=

Description

I find following not clear behaviour of cs2cs during a typing error.

I was surprised, that the declaration with the wrong parameter '+nadgrid=' has another effect as the declaration of the correct parameter '+nadgrids=' with a non existing NTv2 file (see test b+c vs d).

My expectation was, that test b+c+d should use the same default datum shift parameter, but this is not the case.

A description for '+nadgrid=' could not be found in ​ProjAPI. What is the impact of this parameter?